The invention concerns a device for igniting a fuel/air mixture in the combustion chamber of an internal combustion engine, and the device includes an electrode connected to a voltage source and extending into a precombustion chamber. The precombustion chamber is separated at least region-wise from the combustion chamber by a wall, and a control device or regulating device regulates the supply of voltage to the electrode in such a way that a corona discharge takes place in the precombustion chamber. The wall has at least one opening for the exchange of fluids between the combustion chamber and the precombustion chamber. The invention further concerns a method of igniting a fuel/air mixture in the combustion chamber of an internal combustion engine. Finally the invention concerns an internal combustion engine including a device of the aforementioned kind.
In accordance with the state of the art, different ignition concepts are provided for the ignition of Otto engines. At the present time, ignition is effected for the predominant part by means of so-called spark ignition systems which include, for example, a spark plug. Because of constantly increasing demands on efficiency with at the same time low-emission combustion, recourse is increasingly made to alternative ignition technologies. One possibility is represented for example by so-called corona ignition as is described inter alia in U.S. Pat. No. 6,883,507 or FR 2 886 689 A1.
In corona ignition systems, the fuel/air mixture is ignited by high electric field strengths without so-called arc discharge. In contrast to spark ignition systems, in corona ignition only a glow discharge is allowed, but not the production of an avalanche-like arc discharge. Reference is made to arc discharge when a flashover is produced, with the production of an electric ignition spark between the electrodes. In regard to the difference between corona ignition and spark ignition, attention is directed to the relevant literature, for example to the “Encyclopedia of Physics” (Third, Completely Revised and Enlarged Edition, Edited by Rita G. Lerner and George L. Trigg, Volume 1, pages 84 through 89 and pages 394 through 397), the content of which is hereby incorporated and can thus be taken for granted.
The corona ignition system usually employs an electrode in the cylinder head of the internal combustion engine, which is preferably in the form of a tip, to produce high field strengths. Usually the combustion chamber is used as the counterpart electrode. The piston and the underside of the cylinder head form the ground electrode for the corona ignition device.
It has been found that combustion by means of corona ignition is heavily dependent on the combustion chamber geometry, the engine setup and the piston position. To come to grips with those problems, FR 2 886 689 A1 proposes effecting corona ignition in a precombustion chamber by way of which actual ignition is then effected in the combustion chamber. Precombustion chamber ignition is based on the concept that a first fuel/air mixture is ignited in the precombustion chamber and that fuel/air mixture ignited in that way can pass by way of transfer bores into the combustion chamber of the internal combustion engine where ignition of the actual fuel/air mixture takes place. High electric voltages are required to generate the electric field strengths leading to ignition of the fuel/air mixture, which voltages are crucially determined by the electrode arrangement and the mutual spacing of the electrodes. The high voltages however are limited by a number of factors. The voltages have to be generated and transmitted to the electrode in the cylinder head. The space required in the cylinder head and the choice of the insulator material sets limits in regard to the transmission of those high voltages. In the case of very large combustion chambers as occur for example in stationary gas Otto engines which represent a main area of use of the invention, the electrode arrangement in relation to the combustion chamber geometry is problematical as, on the one hand, the piston covers very great distances and, on the other hand, the required voltages are very high due to the spatial arrangement involved. If adjustment of the ignition time is required or if long corona times are necessary for operation of the internal combustion engine, the need for the high voltage varies very greatly.
To implement corona ignition independently of the shape and the piston position it is therefore desirable for the ground electrode to be in the form of a precombustion chamber.
The kind of precombustion chamber ignition described in FR 2 886 689 shows that the fuel is injected into the combustion chamber where it becomes stratified. Injection of the fuel is effected in such a way that a given proportion of the fuel can pass into the precombustion chamber by way of the openings. An internal combustion engine operated in that way tends to have ignition misfires in operation. For example, deposits can block the openings. In the case of conventional spark discharge, such deposits are quickly burnt away. In the case of corona ignition, the deposits are not burnt away because of the lower field strengths. Corona ignition can be operated in a stable fashion only in a narrow band of electric field strength. Deposits which can prevent proper fluid exchange between the precombustion chamber and the combustion chamber can therefore have the result that, because of the inadequate fluid exchange, the voltage regulator re-adjusts the supply of voltage to the electrode. In the extreme case, that can mean that the electrode and the basic highly sensitive electronic components thereof are permanently damaged.